ScTX, the amplitude and V,,, of the action potential develop at a slow rate similar to that of the ... That work demonstrated that during the course sites. In the ...
0270-6474/83/0305-1004$02.00/0 Copyright 0 Society for Neuroscience Printed
DEVELOPMENT DIFFERENTIATION IN CULTURE II. 22Na+ Uptake JESSE
The Journal of Neuroscience Vol. 3, No. 5, pp. 1004-1013
OF SODIUM CHANNELS OF CHICK SKELETAL and Electrophysiological ’ J. BRIAN
*Laboratory of Neurobiology, National Institute of Mental Health, Bethesda, Maryland 20205; *Howard University Center, Department of Oncology, Howard University Medical School, Washington, D. C. 20060; and 0 Laboratory Biochemical Genetics, National Institutes of Health, Bethesda, Maryland 20205 Received
27, 1982; Accepted
Abstract The development of the functional properties of the sodium channel of chick skeletal muscle grown in culture was studied using 22Nauptake and electrophysiological techniques. In accord with the biochemical data in the preceding paper (Baumgold, J., J. B. Parent, and I. Spector (1983) J. Neurosci. 3: 99%1003), the functional manifestations of sodium channel expression are initiated shortly afterace fusion. At this stage, the myotubes have barely detectable sodium-dependent action potentials (V,,, = 1 to 9 V/set) and exhibit a very small amount of batrachotoxin (BTX)-stimulated 22Na+uptake. However, when these cultures are treated with scorpion toxin (ScTX), the amplitude and rate of rise of the sodium action potential increase dramatically ( $‘-, = 35 to 50 V/set) and the (BTX)-stimulated 22Na+ uptake is much larger, suggesting that ScTX unmasks channels that are already present but nonfunctional in these immature myotubes. The two different rates of development of the biochemical properties of the sodium channel described in the preceding paper are also reflected in the two separate rates of development of its functional properties. In the absence of ScTX, the amplitude and V,,, of the action potential develop at a slow rate similar to that of the [3H]saxitoxin binding, eventually reaching a V maxof 158 V/set by day 10; the BTX-stimulated 22Na’ uptake also rises gradually, reaching 12 nmol of 22Na+/culture/min by day 7.5. In contrast, in the presence of ScTX, the Pmaxof the Na+ action potential increases more rapidly, reaching 158 V/set by day 5 and 220 V/set by day 10. The BTX-stimulated 22Na+uptake also increases more rapidly in the presence of ScTX. This rapid rate of development is very similar to that for S~TX binding. These findings and those in the preceding paper suggest the existence of two types of Na+ channels: an immature, nonfunctional channel capable of binding S~TX alone, and a mature, functional channel capable of binding both [‘251]ScTX and [3H]saxitoxin. They further suggest that the insertion of the immature form of the channel protein into the cell membrane shortly after cell fusion is the first event in the expression of the sodium channel. During development, the sodium channel undergoes a structural change which renders it functional. The possibility that both the appearance of functional sodium channels during development and the rapid induction of functional channels by ScTX in immature myotubes reflect a post-translational modification or aggregation of immature nonfunctional channels is discussed.
The preceding paper (Baumgold et al., 1983) described the appearance of two distinct binding components of sodium channels from chick skeletal muscle grown in culture. That work demonstrated that during the course ‘We thank Dr. John Daly, National Institutes of Health (NIH), for providing us with batrachotoxin. This work was supported in part by Grant l-ROl-GM-29804-01 from the NIH. ‘To whom correspondence should be addressed at Laboratory of Neurobiology, National Institute of Mental Health, Building 36, Room
of development of newly fused myotubes, [‘251]scorpion toxin (ScTX)-binding sites appeared first and developed faster than the [3H]saxitoxin ([3H]STX)-binding sites. In the present paper we describe the development of the functional properties of the sodium channel and lD-02, Bethesda, MD 20205. 3Present address: Department ences Center, State University Brook, NY 11790.
of Anatomical of New York
Sciences, Health Sciat Stony Brook, Stony
Development of Sodium Channels
the physiological consequences of ScTX binding using 22Nauptake and electrophysiological measurements. We found that sodium channels develop after fusion with two distinguishable rates: a fast rate observed in muscle cultures treated with ScTX and a slower rate observed in untreated muscle cultures. Based on these results, and those described in the preceding paper, we propose a model for the development of sodium channels and for the effect of &TX on these channels. Parts of this work have been presented in preliminary form (Baumgold et al., 1981; Parent et al., 1981). Materials
and Methods “Na uptake studies. The muscle cells were grown in 35mm tissue culture dishes as described in the preceding paper (Baumgold et al., 1983). The 22Na uptake studies were carried out essentially as described by Catterall (1976). Thus, the growth medium was aspirated and the cells were briefly rinsed with toxin incubation medium (130 mM choline chloride, 50 mM HEPES-Tris, pH 7.4, 5.5 mM glucose, 0.8 mu MgS04, 5.4 mM KCl, 1 mg/ml of bovine serum albumin (BSA)), then incubated for 30 min at 37’C with the same medium containing the indicated toxins. This medium was then replaced with a sodium uptake medium containing 120 mM choline chloride, 10 mM NaCl, 50 mM HEPES-Tris, pH 7.4, 5.5 mM glucose, 0.8 mM MgS04, 5.4 mM KCl, 5 mu ouabain, and 1.3 PCi of 22Na (New England Nuclear) per ml. After a 1-min incubation at room temperature, the cells were washed five times (total washing time was less than 10 set) with 2.5 ml of wash medium containing 163 mM choline chloride, 5 mM HEPES-Tris, pH 7.4, 1.8 mM CaC12,0.8 mM MgSO+ and 1 mg/ml of BSA. The cells were harvested DAY 2%
in 0.4 N NaOH and counted in a Beckman 4000 gamma counter at a counting efficiency of 85%. Electrophysiology. The electrophysiological experiments were performed on cultures maintained at 35 to 38°C with techniques similar to those previously described (Spector and Prives, 1977). During the experiments, the cells were bathed in a HEPES-buffered (pH 7.3) modification of Eagle’s medium (Flow Laboratories, Inc.) which had the following ionic composition: 116 mM NaCl, 5.4 mu KCl, 1.8 mM CaC12,0.8 mu MgSO+ and 1 mM NaH2P04. Cells were impaled by a single microelectrode filled with 3 M KC1 (resistance 10 to 20 megohms), which was used for both intracellular current injection and voltage recording. Results Development during muscle
of neurotoxin-stimulated differentiation. Mature
SCTX BTX SCTX BTX
myotubes have been shown to accumulate 22Na+ in a linear fashion for several minutes in response to stimulation by alkaloid neurotoxins such as batrachotoxin (BTX) or veratridine (Frelin et al., 1981). Thus, the amount of 22Na’ taken up in 1 min is a good measure of the initial rate of 22Na+ uptake in these cultures. In order to determine the development of BTX sensitivity during muscle differentiation, we measured the initial rate of 22Na+ uptake in BTX-stimulated cultures of chick skeletal muscle of various ages. As shown in Figure 1, the initial rate of 22Na+ uptake in 2.5-day-old cultures in response to BTX (200 nM) was 1.3 nmol of 22Na/min/culture, only slightly higher than that of control cultures. By the following day, however, cultures exposed to BTX had an initial rate of 22Na+uptake double that of control cultures and
SCTX BTX SCTX
SCTX BTX SCTX Bk
1. Effect of various neurotoxins on the **Na uptake of musclecells grown in culture for various lengths of time. The cells
were plated at an initial density of 4.2 X lo4 cells/cm2 as described under “Materials and Methods.” The uptake studies were performed, as describedunder “Materials and Methods,” using 300nM scorpion toxin (ScTX) and 200nM batrachotoxin (BTX). The background radiation has been subtracted from all values. C representsthe amount of **Na+taken up in the absenceof any neurotoxin. The values shown are the average of three determinations and the error bars represent the standard deviation.
accumulated about 4.5 nmol of 22Na’/min/culture. This BTX sensitivity gradually increased further during development and reached a maximum of approximately 12.5 nmol of 22Na+/culture/min by day 7.5. Control cultures, in contrast, showed only a slight increase in their initial rate of 22Na+uptake, reaching a value of about 3.5 nmol of 22Na+/culture/min by day 7.5. This gradual increase in the initial rate of 22Na+uptake in response to BTX stimulation is shown by the open circles in Figure 2. This figure demonstrates that as the cultures mature, their initial rates of 22Na+ uptake increase linearly with time between days 2.5 and 7.5 in culture, suggesting that BTX-sensitive sodium channels first appear in 2.5-dayold cultures and increase in number over the next 5 days. Similar experiments designed to assessthe ScTX sensitivity of these cultures did not reveal any increase in the initial rate of “Nat uptake above control levels between 2.5 and 7.5 days in culture (Fig. l), suggesting that under the conditions of these experiments, ScTX alone (300 mu) was unable to cause steady influx of Na+ ions. However, in the presence of BTX (200 nM), ScTX increased the initial rate of 22Na+uptake to a value that was considerably higher than that for BTX alone. This cooperative interaction between BTX and ScTX has been described previously (Catterall, 1980) and was already demonstrable in 2.5-day-old cultures, which exhibited an initial rate of 22Na+ uptake of about 2.3 nmol of 22Na+/min/culture. This value increased rapidly between days 3 and 4.5 in culture, reaching 17 nmol of 22Na+/min/culture, then continued to increase at a slower rate for the following few days (Fig. 2, solid
circles). As shown in Figure 2, the slower increase in the initial rate of “Na+ uptake in response to both toxins
Vol. 3, No. 5, May 1983
(solid circles) after day 4 in culture approximately parallels the increase in the initial rate of 22Na+ uptake in response to BTX alone (open circles). The 22Na+uptake stimulated by BTX (or veratridine) alone and that stimulated by BTX (or veratridine) plus ScTX were sensitive to tetrodotoxin (TTX) in a dosedependent manner. As shown in Figure 3, mature (g-dayold) cultures exhibited a dose dependency for TTX which had a half-maximal effect at 4 nM TTX, a value that is in close agreement with the & of binding of [3H]TTX or [3H]STX to chick skeletal muscle (Frelin et al., 1981; Baumgold et al., 1983). The immature (day 4) cultures, however, were considerably less sensitive to TTX than the mature cultures, and exhibited a half-maximal effect at 15 nru TTX. It is unclear why the immature cultures have a & for [3H]STX binding that is similar to that of mature cultures (Fig. 5, Baumgold et al., 1983), but are less TTX sensitive than mature cultures, as measured either electrophysiologically (see below) or by 22Na+uptake studies (Fig. 3). Electrophysiology. Under the culture conditions used in the present study, the rapid burst of cell fusion (30 to 50 br after
by a period
myotube growth. By 72 hr, the cultures consisted predominantly of large syncytial myotubes which had an average resting potential (RP) of about -65 mV. This large RP remained constant for the next 8 days in culture (Fig. 4B), thus making it possible to study the development of the Na+ conductance and the effects of ScTX on this development without adjustments of the RP. Development of the Na’ conductance during myotube differentiation. As a result of different culture conditions, the initial and final events in the developmental time
I I I II
z5 609ii ? f
. BTX + ScTX xFUSlON INDEX
: AX OO
4 6 DAYS IN CULTURE
Figure 2. Time course for the development of BTX (O)- and BTX + &TX (.)-stimulated ‘“Na+ uptake. The same data as shown in Figure 1 apply, except that the amount of 22Na+ taken up in the absence of any drug (C, Fig. 1) has been subtracted from all values and the amount of “Na+ taken up by 7.5-day-old cultures in the presence of BTX + ScTX has been defined as 100%. BTX (200 nM) and ScTX (300 nM) were used. The values shown are the average of three determinations and the error bars represent the standard deviation. x represents the fusion index which was determined as described under “Materials and Methods.”
m-xl Figure 3. TTX sensitivity of veratridine (200 pM) plus ScTX (100 nM)-stimulated “Na+ uptake in muscle cells grown in culture. The cells were plated at an initial density of 4.2 x lo4 cells/cm’. The uptake study was performed as described under “Materials and Methods” with the exception that the indicated concentrations of TTX were added to the toxin incubation medium. The cells were incubated in the 22Na+ uptake medium for 30 sec. The amount of 22Na+ taken up in the absence of drugs was subtracted from all values. Each point represents the average of three determinations whose standard deviation was usually less than 10%.
DAYS IN CULTURE Figure 4. A, Changes in the maximum rate of rise (vm,,) of the Na+ action potential during differentiation of chick skeletal muscle in culture. 0, control cultures; 0, cultures treated with 216 nM ScTX. Eachpoint representsmeasurementsfrom 20 to 25 large myotubes that displayed a maximum action potential at the site of recording. Myotubes in 5- to &day-old cultures that displayed passive (i.e., Fig. 5C) or small (i.e., Fig. 6A) v,,, at the site of recording were discarded. B, Changes in the resting potential as a function of time in culture.
course of the Na+ conductance were somewhat different from those described previously (Spector and Prives, 1977). Thus, depolarizing pulses applied from the RP of long or branched myotubes in day 3 cultures elicited only
slow (maximum rate of rise